New Feature: Burst Detection

I implemented Burst Detection,
which takes strings with time-points and detects temporary increase of specified keyword's frequency.

~~**This pull-request is WIP; DO NOT MERGE yet.**~~

Features to be implemented:
- [x] `get_status`
- [x] distribution logic

**All Works done!**
MSGPACK_DEFINE is used in this header, so ```#include <msgpack.hpp>``` or ```#include <msgpack/type/define.hpp>``` is required.